On Jun 1, 2007, at 8:24 AM, Tasneem Memon wrote:
NEAR
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It deals with the NUMBER and DATE datatypes simulating the human
behavior and processing the
Why just number and date?
information contained in NEAR in the same way as we humans take it.
This is a binary operator with the syntax:
op1 NEAR op2
Here, the op1 refers to an attribute, whereas op2 is a fixed value,
both of the same datatype.
Suppose we want a list of all the VGAs, price of which should be
somewhere around 30$ .. the query will look like:
SELECT *
FROM accessories
WHERE prod_name = ‘VGA’
AND prod_price NEAR 30
A query for the datatype DATE will look like:
SELECT *
FROM sales
WHERE item = ’printer’
AND s_date NEAR 10-7-06
The algorithm for the NEAR operator works as follows:
The margins to the op2, i.e. m1 and m2, are added dynamically on
both the sides, considering the value it contains. To keep this
margin big is important for a certain reason discussed later.
The NEAR operator is supposed to obtain the values near to the op2,
thus the target membership degree(md) is initially set to 0.8.
The algorithm compares the op1(column) values row by row to the
elements of the set that NEAR defined, i.e. the values from md 1.0
to 0.8, adding matching tuples to the result set.
How would one change 0.8 to some other value?
4. It is very much possible that the result set is empty since
no values within the range exist in the column. Thus, the algorithm
checks for empty result set, and in that case, decreases the target
md by 0.2 and jumps to step 3. This is the reason big margins to
the op2 are added.
5. In case there are no values in op1 that are between m1 and
m2 (where the membership degree of the values with respect to NEAR
becomes 0.1) and the result set is empty, the algorithm fetches the
two nearest values (tuples) to op2, one smaller and one larger than
the op2, as the result.
The algorithm will give an empty result only if the table referred
to in the query is empty.
2. NOT NEAR
This operator is also a binary operator, dealing with
the datatype NUMBER and DATE. It has the syntax:
op1 NOT NEAR op2
The op1 refers to an attribute, whereas op2 is a fixed value, both
of the same data type.
A query containing the operator looks like:
SELECT id, name, age, history
FROM casualties
WHERE cause = ‘heart attack’
AND age NOT NEAR 55
Or suppose we need a list of some event that is not clashing with
some commitment of ours:
SELECT *
FROM events
WHERE e_name= ‘concert’
AND date NOT NEAR 8/28/2007
The algorithm for NOT NEAR works like this:
First of all it adds the margins to the op2, i.e. m1 and m2,
dynamically on both the sides, considering the value op2 contains.
op1 values outside the scope of the op2 (m1, m2) are retrieved and
added to the result.
If the result set is empty, the farthest values within the op2
fuzzy set (those possessing the least membership degree) are
retrieved. This is done by continuing the search from values with
md=0.1 till the md=0.6, where the md for NOT NEAR reaches 0.4.
Why isn't this just the exact opposite set of NEAR?
--
Jim Nasby [EMAIL PROTECTED]
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
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